Interdigitated back contact metal-insulator-semiconductor solar cell with printed oxide tunnel junctions

ABSTRACT

Screen-printable metallization pastes for forming thin oxide tunnel junctions on the back-side surface of solar cells are disclosed. Interdigitated metal contacts can be deposited on the oxide tunnel junctions to provide all-back metal contact to a solar cell.

This application is a division of U.S. application Ser. No. 16/857,627,filed on Apr. 24, 2020, now allowed, which is a division of U.S.application Ser. No. 16/003,506, filed on Jun. 8, 2018, and issued asU.S. Pat. No. 10,670,187, which is a continuation of U.S. applicationSer. No. 15/839,585, filed on Dec. 12, 2017, and issued as U.S. Pat. No.10,026,862, which is a division of U.S. application Ser. No. 15/663,187,filed on Jul. 28, 2017, and issued as U.S. Pat. No. 9,929,299, which isa continuation of PCT International Application No. PCT/CN2016/111035,filed on Dec. 20, 2016.

FIELD

The invention relates to screen-printable metallization pastes forforming thin oxide tunneling layers on the back-side surface of solarcells. Interdigitated metal contacts can be deposited on the oxidelayers to provide all back metal contact to a solar cell.

BACKGROUND

It is desirable to increase the efficiency of silicon solar cells inorder to produce more power and reduce the cost of generating solarenergy per unit area. One of the most effective ways to improve solarcell efficiency is to situate all electrical contacts on the backsurface of the solar cell away from the incident solar radiation.Without a front-side metallization grid the entire top surface area ofthe solar cell can absorb incident solar radiation. An example of thistype of solar cell is the interdigitated back-contact solar cell, whichcan yield median solar cell efficiencies greater than 23%. However, theprocess for fabricating interdigitated back-contact solar cells is morecomplex and expensive compared to conventional solar cell fabricationprocesses with front-side metallization grid and a back-side aluminumback-surface field.

SUMMARY

According to the invention, a method of interconnecting a siliconsubstrate, comprises forming a positively charged oxide tunnel junctionon a first region of a back surface of a silicon substrate; forming anegatively charged oxide tunnel junction on a second region of a backsurface of a silicon substrate; forming a positive electrode on thepositively charged oxide tunnel junction; and forming a negativeelectrode on the negatively charged oxide tunnel junction.

According to the present invention, a solar cell comprises interconnectsis fabricated using the method according to the present invention.

According to the present invention, a solar cell comprises a backsurface comprising a first region and a second region; apositively-charged silicon oxide layer overlying the first region; anegatively-charged aluminum oxide layer overlying the second region; apositive electrode grid overlying the positively-charged silicon oxidelayer; and a negative electrode grid overlying the negatively-chargedaluminum oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art will understand that the drawings describedherein are for illustration purposes only. The drawings are not intendedto limit the scope of the present disclosure.

FIG. 1A shows a cross-sectional view of a silicon wafer with texturedfront and back surfaces.

FIG. 1B shows a cross-sectional view of a silicon wafer with aphosphorous diffusion layer on the top textured surface of the siliconwafer.

FIG. 1C shows a cross-sectional view of a silicon wafer with anantireflection layer overlying a phosphorous diffusion layer on the toptextured surface of the silicon wafer.

FIG. 1D shows a cross-section view of the silicon wafer of FIG. 1C afterplanarization of the back-side textured surface.

FIG. 1E shows a cross-sectional view of the planarized silicon wafer ofFIG. 1D after printing a positively-charred oxide patterned layer on theplanarized back surface of the silicon wafer.

FIG. 1F shows a cross-sectional view of the silicon wafer of FIG. 1Eafter printing a negatively-charged oxide patterned layer on theplanarized back surface of the silicon wafer.

FIG. 1G shows a cross-sectional view of the silicon wafer of FIG. 1Fwith printed positive and negative electrode grids.

FIG. 2 shows a back surface electrode pattern.

FIG. 3 shows a schematic of an energy band diagram for an all backcontact metal-insulator-semiconductor solar cell.

DETAILED DESCRIPTION

There is a need to improve the efficiency of silicon solar cells whilemaintaining the cost per Watt of the photovoltaic module as low aspossible. This requires novel processes, material development andtechniques to reduce the fabrication cost of the all-back contact solarcells.

All back surface contact solar cells can be fabricated using printeddielectrics on the back-side of the solar cell to act as passivating andinsulating tunneling layers for current collection to form ametal-insulator silicon semiconductor (MIS) back-contact solar cell.This fabrication method can eliminate several costly and time-consumingfabrication steps typically used to fabricate all back-contact siliconsolar cells. Using the methods disclosed herein, the number of steps andcost of materials used can be similar to that of conventional siliconsolar cells.

Metal-insulator-semiconductor (MIS) solar cells can be fabricated usingscreen printed dielectrics and interdigitated back surface contacts.Screen printed metallization pastes can be used to provide n+- andp+-doped tunneling regions on the back surface of a solar cell.Interdigitated contacts can be formed by screen printing metallizationpastes over the tunneling regions. The use of screen printing isconsistent with low cost solar cell manufacturing methods.

MIS oxide tunnel junctions for back-side solar cell interconnection suchas aluminum oxide thin films are typically deposited by atomic layerdeposition. Thin silicon oxide films can be formed by thermal oxidation,wet or steam oxidation, or by plasma-enhanced chemical vapor deposition(PECVD). With these deposition methods masking and etching techniquesare used after deposition to pattern the back surface for an allback-contact Si solar cell. Alternatively, using masks during oxidedepositions for depositing patterned oxides can be incompatible withhigh-volume, low-cost manufacturing due to the difficulty in maintainingcontamination-free surfaces as well as added overhead cost related tosuch techniques.

The use of screen printing to form the ultra-thin oxide dielectriclayers and the electrodes is compatible with high-volume manufacturing.

Printed back-side MIS solar cells can be fabricated using the followingprocess.

A textured silicon wafer, either N-type or P-type, can be cleaned usinga suitable wet-chemical procedure, rinsed and dried. For an N-type Sisolar cell using POCl₃ or other phosphorous source, phosphorous can bediffused into the top surface of the silicon wafer to provide afront-surface field. The silicon wafer is then deglazed in ahydrofluoric acid solution. After deglazing, an antireflection coating,such as a SiN_(x) dielectric layer is deposited over thephosphorous-diffused layer on the front-side of the wafer.

Texture on the back-side of the silicon wafer can be removed using asuitable etching solution such as a concentrated alkaline etchingsolution of sodium hydroxide or potassium hydroxide, to provide a planarback-side surface. The front-side antireflection coating and passivationlayer serves as a barrier to etching of the front-side surface.

The silicon wafer can then be cleaned in a hydrochloric acid solution.The native oxide on the back-side surface may be removed using abuffered oxide dip.

A positively charged p+-doped silicon oxide film can be screen printedonto regions of the back-side surface and dried in a reducing or inertatmosphere. In other regions of the back-side surface, a negativelycharged n+-doped oxide film can be screen printed and dried. Theunderlying p- and n-regions of the Si semiconductor can form aninterdigitated pattern or any other suitable pattern for the collectionof electrons and holes.

The screen printed thin-film oxides can be heated to a temperaturesufficient to volatilize and/or to decompose organic components of thedielectric pastes used to form the thin film oxides; to create localtunnel junctions; and to cure the dielectric pastes. For example, thedielectric thin films can be heated to 35° C., to 400° C., or to 450° C.The final thickness of the thin-film oxide can be, for example, lessthan 100 Å, less than 50 Å, less than 30 Å, less than 20 Å, or less than10 Å.

After the thin film oxide tunnel junctions have been formed, positiveand negative electrodes can be screen-printed onto the p+ and n+ metaloxide tunnel junctions. The positive and negative electrodes can beapplied by screen printing in a single step or in more than one step.

The metallization paste can cure at temperatures such as less than 500°C., less than 450° C., or less than 400° C. A metallization paste cancomprise an amorphous metal glass to provide for a uniform ohmic contactto the tunnel junctions without etching or damaging the tunnel junctionsduring the high temperature cure. A metallization paste can comprise,for example, copper particles, copper alloy particles, silver particles,silver alloy particles, or a combination of any of the foregoing.

Examples of steps used in the fabrication of printed interdigitated backmetal contact MIS solar cells is shown in FIGS. 1A-1G.

FIG. 1A shows a cross-section of a textured N-type silicon wafer 101including a top (front-side) textured surface 102 and a bottom(back-side) textured surface 103.

FIG. 1B shows a cross-section of a textured silicon wafer 101 afterphosphorous diffusion on the front-side surface 102 to form an n+-dopedlayer 104 having a high sheet resistance.

FIG. 1C shows a cross-section of a textured silicon wafer 101 after adielectric antireflection coating 105 such as silicon nitride is appliedover the n+ doped surface layer 104.

FIG. 1D shows a cross-section of a silicon wafer 101 after the back-sidesurface 103 has been planarized.

FIG. 1E shows a cross-section of a silicon wafer 101 after a positivelycharged oxide layer 106 has been applied to regions of the back-side byscreen printing and curing. The positively charged oxide dielectriclayer can comprise a positively charged silicon oxide layer. The curedthickness of the positively charged silicon oxide layer can be, forexample, less than 50 Å.

FIG. 1F shows a cross-section of a silicon wafer 101 after a negativelycharged oxide layer 107 such as a negatively charged aluminum oxidelayer is applied in the back-side regions not covered by the positivelycharged oxide layer 106. The negatively charged oxide layer can beapplied by screen printing.

FIG. 1G shows a cross-section of a silicon wafer 101 afterinterdigitated gridlines 108/109 are applied to the positively andnegatively charged oxide tunnel junctions by screen printing. Positiveelectrodes 108 are electrically interconnected to positively chargedoxide tunnel junctions 106 and negative electrodes 109 are electricallyinterconnected to negatively charged oxide tunnel junctions 107.

FIG. 2 shows a view of the back-side surface of the silicon wafershowing an example of a configuration for the interdigitated gridlinesincluding positive electrodes 201 and negative electrodes 202.

An energy band diagram for the solar cell structure is shown in FIG. 3 .From left to right the diagram shows the energy bands for the positivelycharged oxide layer OX(+ve), the n-type silicon substrate N—Si, the“virtual” p-n junction, and the negatively charged oxide layer OX(−ve).

The gridline metallization paste can uniformly contact the oxide tunneljunction and will not etch through the oxide layers during development.

A gridline metallization paste can comprise an electrically conductivematerial such as silver, copper, silver alloy, copper alloy, or acombination of any of the foregoing. Silver and copper is intended toinclude the pure metal and alloys of the metal. For example, a gridlinemetallization paste can comprise copper, a copper alloy, or acombination thereof.

The electrically conductive particles can comprise, for example, copperand/or copper alloy particles that have been coated with a metallicglass.

The gridline metallization paste can comprise a metallic glass.

Metallic glasses have previously been proposed for use in solar cellelectrodes. For example, metallic glass has been proposed for used asfrit in thick-film Ag metallization pastes for making electrical contactto bare silicon or silicon oxide surfaces in solar cells. Kim et al.,Scientific Reports 3:2185, DOI: 10.1038/srep02185. Conventional Agmetallization pastes incorporating oxide glasses can flow and dissolvethe silicon oxide tunnel junction and the underlying silicon to a depthof 1,000 A or more. Also, traditional metal oxide glass frit technologyhas only been shown to work with Ag as the functional phase powdermaterial.

On the other hand, metallic glass will not etch through typicaldielectric layers such as silicon nitride layers used in solar cells andwill not flow at temperatures in solar cell manufacturing. At solar cellmanufacturing temperatures, metallic glass will crystalize therebyimproving the electrical conductivity. Thus, metallic glass electrodeswill not damage the oxide and oxide-silicon interface and will notcompromise the surface recombination velocity of the solar cell.

The metallic glass can also act as a barrier to copper diffusion intosilicon, thereby enhancing the reliability of the solar cell.

During annealing of the solar cell, a metallic glass can be heated to atemperature that is higher than the glass transition temperature of themetallic glass such that during cooling the metallic glass can at leastpartly crystallize, which can increase the mechanical stability of themetallic glass and lead to increased conductivity.

A metallic glass can comprise at least two elements. For example, afirst element of the at least two elements can have a high electricalconductivity and can have a higher crystallization temperature than theother elements forming the metallic glass.

A metallic glass can comprise, for example, silver (Ag), copper (Cu),gold (Au), aluminum (Al), calcium (Ca), beryllium (Be), magnesium (Mg),sodium (Na), molybdenum (Mo), tungsten (W), tin (Sn), zinc (Zn), nickel(Ni), potassium (K), lithium (Li), iron (Fe), palladium (Pd), platinum(Pt), rubidium (Rb), chromium (Cr), strontium (Sr), zirconium (Zr),cobalt (Co), hafnium (Hf), titanium (Ti), manganese (Mn), iron (Fe),phosphorus (P), ruthenium (Ru), yttrium (Y), lanthanum (La), niobium(Nb), neodymium (Nd), vanadium (V), boron (B), silicon (Si), osmium(Os), gallium (Ga), or a combination of any of the foregoing.

Examples of suitable metallic glass alloys include ZrCuNiAl, CuZr,CuZrAl, CuZrAg, CuZrAlAg, CuZrAlAgNi, CuZrAlNi, ZrTiNbCuNiAl, MgZn,MgCu, MgNi, MgNiY, MgCa, AlNiYLa, AlMg, AlMgCa, TiZrCuNi, NiNbZrTiPt,AlNiLa, ZrCuTiNiBe, MgCuYAg, AlLiCu, AlYFe, AgMgCa, and AgMgCaCu. Ametallic glass can comprise copper, zirconium, aluminum, silver, nickel,or a combination of any of the foregoing.

Metallic glasses can be formed by heating a combination of elements toform a metal liquid and rapidly quenching the liquid.

A metallic glass can include an alloy having a disordered atomicstructure comprising two or more elements. A metallic glass may be ametallic glass or may be at least partly crystallized. A metallic glassmay have from 50 wt % to about 99.9 wt %, such as from 60 wt % to 99 wt%, or from 70 wt % to 95 wt % amorphous content, based on a total weightof the metallic glass. A metallic glass may comprise from 1 wt % to 50wt %, such as from 2 wt % to 40 wt %, or from 4 wt % to 30 wt %crystalline content, based on a total weight of the metallic glass.

A metallic glass can be characterized by a low resistance. For example,a metallic glass may have a resistivity within a range from 2 μΩ-cm to1000 μΩ-cm, such as within a range from 5 μΩ-cm to 800 μΩ-cm, or from 10μΩ-cm to 600 μΩ-cm. The resistivity of the metallic glass can decreasewhen the metallic glass is heat treated at a temperature that is higherthan a glass transition temperature of the metallic glass. Thetemperature that is higher than a glass transition temperature of themetallic glass may be within a range from 1° C. to 300° C., such aswithin a range from 5° C. to 250° C., or within a range from 10° C. to200° C. higher than the glass transition Tg of the metallic glass.

For example, when heat-treated at a temperature within a range from 400°C. to 800° C., such as within a range from 500° C. to 700° C., theresistivity of the metallic glass may decrease by 1 μΩ-cm to 200 μΩ-cm,such as by 5 μΩ-cm to 150 μΩ-cm, by 10 μΩ-cm to 100 μΩ-cm, or by 20μΩ-cm to 75 μΩ-cm. In comparison, metal oxide glass metallization pastescomprising Ag particles, can have a high resistivity greater than about10¹³ Ω-cm, which after sintering of the Ag particles can be reduced to10⁷ Ω-cm to 10⁹ Ω-cm. Amorphous and crystallized metallic glass can havea much lower resistivity than conventional glasses in typicalmetallization pastes.

Because the metallic glass has a low resistivity it may be considered tobe an electrical conductor at a voltage and at a current of a solarcell.

A metallic glass may be characterized by a glass transition temperature,Tg, for example, greater than 100° C., greater than 150° C., or greaterthan 200° C. A metallic glass can be characterized by a glass transitiontemperature within the range from 100° C. to 700° C., such as within therange from 150° C. to 650° C., or within the range from 200° C. to about600° C. A metallic glass can be characterized by a crystallizationtemperature Tc within a range from 120° C. to 720° C., such as withinthe range from 170° C. to 670° C., or within the range from 220° C. to620° C.

A metallic glass alloy can be selected to exhibit a suitablecrystallization temperature. For example, a suitable metallic glassalloy can have a crystallization temperature less than that of thefiring temperature used to develop the metallization paste used to formthe electrical conductors. A firing temperature can, for example, beless than 600° C., less than 500° C., less than 450° C., or less than400° C.

A metallic glass alloy can be selected to exhibit a suitable adhesion tosilicon oxide and/or to a metal forming a wire core. For example, ametallic glass alloy can be selected to have a pull strength on a solarcell surface from 1 N/mm to 7 N/mm, as determined according to a 180°pull test with a 50 mm/min stretch rate.

A metallic glass alloy can be selected to have a suitable electricalresistivity such as from 50 μΩ-cm to 20,000 μΩ-cm.

The positive and negative electrodes of an all-back contact solar cellcan be fabricated using metallization pastes comprising metallicglass-coated particles.

Solar cell thick film metallization pastes typically contain Agparticles. Because of the much lower costs, it is desirable to usecopper as the conductive metal. However, thick film metallization pastesusing glass frit to etch through thick oxide layers has only beendemonstrated to work effectively with Ag, in part due to the propensityfor copper to diffuse into the silicon. Copper will also tend to oxidizeduring firing, which significantly reduces the sintered copper lineconductivity.

Because the tunnel junctions are exposed, it is possible to employmetallization pastes that do not comprise glass frit to form thepositive and negative electrodes.

Aspects of the invention include thick film metallization pastescomprising metallic glass-coated particles. The metallic glass-coatedparticles can comprise a core of a conductive metal or metal alloysurrounded by a coating of a metallic glass. The electrically conductivecore material can be, for example, silver, a silver alloy, copper, or acopper alloy. Metallization pastes comprising metal-coated copper alloyparticles have been proposed, for example see U.S. ApplicationPublication No. 2011/0315217, which discloses copper particles coatedwith one or more layers of metal.

Aspects of the present invention also include silver particles having acoating of a metallic glass. As used herein, silver refers to puresilver and silver alloys. Metallic glass-coated silver particles canhave the same or similar properties, dimensions, and compositions as formetallic glass-coated copper particles. Metallic glass-coated silverparticles may be used in the same way as metallic glass-coated copperparticles to form a metallization paste.

A metallization paste and resulting solar cell electrode may includemetallic glass-coated copper particles, metallic glass-coated silverparticles, or a combination thereof. The particles used in ametallization paste may have a coating of the same or similar metallicglass or may have coatings of different metallic glasses. For example,for copper particles, it may be useful that the metallic glass coatingbe selected to prevent or minimize copper diffusion. The metallic glasscoating the particles can include any of the metallic glasses referredto herein for coating a wire conductor.

Metallic glass-coated particles can be incorporated into a suitablemetallization paste, which can include various components forformulating the desired properties of the paste. Upon sintering themetallic glass coating can at least partly crystallize to increase theconductivity of the metallic glass coating and help sinter adjacentparticles to enhance the conductivity of the electrode. The metallicglass coating can provide a copper and/or silver diffusion barrier andcan prevent or substantially reduce corrosion of the copper and/orsilver particle.

A metallization paste may comprise metal particles and metallicglass-coated particles; metal particles and metallic glass; or metallicglass-coated particles and metallic glass. For example, a metallizationpaste may comprise silver and/or copper particles, and metallicglass-coated silver and/or copper particles; silver and/or copperparticles and metallic glass; or metallic glass-coated silver and/ormetal particles and metallic glass.

The metallic glass-coated particles can be characterized by an averageparticle diameter, for example, from 1 nm to 1000 nm, from 1 nm to 600nm, from 1 nm to 400 nm, from 20 nm to 400 nm, or from 50 nm to 200 nm.The metallic glass-coated particles can be characterized averageparticle diameter D50 from 1 μm to 200 μm, from 1 μm to 150 μm, from 1μm to 100 μm, from 1 μm to 50 μm, from 1 μm to 30 μm, or from 1 μm to 20μm.

A metallization paste may also include other metal particles in additionto the metallic glass-coated particles. The additional metals can beused to adjust the rheology, bonding, adhesion, fusion during firing orsintering, and/or electrical conductivity. For example, the additionalparticles can include Ag particles. A metallization paste may comprisecopper and/or silver particles coated with different metallic glasses.The different metallic glass may be selected, for example, based onelectrical conductivity, melt temperature, or crystallizationtemperature. For example, a certain metallic glass may have a lower melttemperature, which could enhance binding, but also exhibit a lowerconductivity. Particles with this metallic glass coating could becombined, for example, with copper particles having a high electricalconductivity.

In certain embodiments, a metallization paste may comprise silverparticles, without frit and without metallic glass. Electricallyconductive silver pastes capable of curing at low temperature are knownin the art. Any suitable metallization paste comprising silver particlesused to form solar cell electrodes may be used.

Conductive particles such as copper and/or silver particles can becoated with a metallic glass. The particles can be combined with otheradditives, modifiers, and organic medium to form a metallization pastesimilar to a Ag metallization paste. The metallization paste containingmetallic glass-coated particles can then be applied or printed on asurface of a solar cell having openings exposing localized BSF regions(back-side) or emitter regions (front-side). As with the metallicglass-coated wires, the metallic glass will not damage the tunneljunctions and upon annealing can form highly conductive electrodes.

A metallization paste may further include additives to modify thephysical properties of the paste such as to enhance flow, processproperties, and stability. Additives may include, for example,dispersants, thixotropic agents, plasticizers, viscosity stabilizers,anti-foaming agents, surfactants, pigments, UV stabilizers,antioxidants, coupling agents, and combinations of any of the foregoing.

A metallization paste provided by the present disclosure can comprise,for example, from 0.01 wt % to 5 wt % of an organic resin; from 1 wt %to 45 wt % of a solvent; and from 0.01 wt % to 5 wt % of one or moreadditives, where wt % is based on the total weight of the composition.

A composition can comprise an organic binder or combination of organicbinders.

An organic binder, also referred to as an organic resin, can be used toimpart a desired viscosity and/or rheological property to ametallization paste to facilitate screen printing solar cell electrodes.The organic binder can also facilitate homogeneous dispersion of theinorganic component within the printable composition.

Suitable organic binders include, for example, acrylate resins andcellulose resins such as ethylcellulose, ethyl hydroxyethylcellulose,nitrocellulose, blends of ethylcellulose and phenol resins, alkydresins, phenol resins, acrylate esters, xylenes, polybutanes,polyesters, ureas, melamines, vinyl acetate resins, wood rosin,polymethacrylates of alcohols, and combinations of any of the foregoing.

Other suitable resins include, for example, ethyl cellulose, celluloseester (CAB, CAP), polyacrylate, polysiloxane (modified), polyvinylbutyral (PVB), polyvinyl pyrrolidone (PVP), saturated polyester,non-reactive polyamide (PA), modified polyether, and combinations of anyof the foregoing. Other resins characterized by medium polarity may alsobe used. A resin can comprise ethyl cellulose.

An organic binder may be present in an amount from 0.1 wt % to 10 wt %,from 0.1 wt % to 6 wt %, from 0.2 wt % to 4 wt %, from 0.2 wt % to 2 wt%, or from 0.2 wt % to 1 wt %, where wt % is based on the total weightof the printable composition.

A composition can comprise an organic solvent or combination of organicsolvents.

An organic solvent can be used to impart solubility, dispersion, andcoupling to the metallization paste.

Examples of suitable solvents include terpineol, glycol ether, glycolether acetate, Texanol™ (ester alcohol), tributyl citrate, tributylO-acetylcitrate, DBE® esters (mixture of dimethyl adipate, dimethylglutarate and dimethyl succinate); dimethyl phthalate (DMP), andcombinations of any of the foregoing. A suitable solvent can have, forexample, a boiling point greater than 200° C. and an evaporation rateless than 0.01 at room temperature. A suitable solvent can be anoxygenated solvent including alcohols such as ethanol, methanol,butanol, n-propyl alcohol, isobutyl alcohol, and isopropyl alcohols;esters such as ethyl acetate, n-butyl acetate, n-propyl acetate, andisopropyl acetate; and ketones such as acetone, diacetone alcohol,isophorone, cyclohexanone, methyl ethyl ketone, and methyl isobutylketone. Other suitable ethers, alcohols, and/or esters may also be used.

In certain embodiments, a solvent comprises a glycol ether.

Other examples of suitable solvents include hexane, toluene, ethylCellusolve™, cyclohexanone, butyl Cellusolve™, butyl carbitol(diethylene glycol monobutyl ether), dibutyl carbitol (diethylene glycoldibutyl ether), butyl carbitol acetate (diethylene glycol monobutylether acetate), propylene glycol monomethyl ether, hexylene glycol,terpineol, methylethylketone, benzyl alcohol, γ-butyrolactone, ethyllactate, and combinations of any of the foregoing.

A printable composition can include from 1 wt % to 15 wt %, from 2 wt %to 10 wt %, from 3 wt % to 9 wt %, or from 5 wt % to 8 wt % of anorganic solvent, where wt % is based on the total weight of theprintable composition.

An additive or combination of additives may be present in thecomposition in an amount, for example, from 0.1 wt % to about 5 wt %,from 0.1 wt % to 1.5 wt %, from 0.5 wt % to 1.5 wt % or from, 0.3 wt %to 1 wt %, where wt % is based on the total weight of the composition.

For screen printing fine lines with a high aspect ratio it is desirablethat a front-side metallization paste provided by the present disclosureexhibit a viscosity, for example, of 500 Poise to 7000 Poise at atemperature from 15° C. to 50° C., as determined using a viscometer witha 10 rpm spindle rotation rate.

It can also be desirable that a metallization paste exhibit a glasstransition temperature T_(g) from 200° C. to 800° C. as determined usingdifferential scanning calorimetry (DSC).

A metallization paste can be prepared using the following procedure.

An organic vehicle can be prepared by mixing and heating a solvent ormixture of solvents and organic resin or organic resins, plasticizer,defoaming agent, and additives such as rheological thixotropic additive.

Metallic glass-coated particles can be combined with the organicvehicle, organic vehicle and other additives and thoroughly mixed.

The metallization paste can then be milled to achieve a desireddispersion of the inorganic components. The metallization paste can thenbe filtered to remove any undesired large particulates.

The metallization paste can be applied to a surface of a silicon solarcell by screen printing. The screen used in solar cell screen printingcan be a mesh covered by an emulsion which is patterned to form the gridpattern. The mesh number can be, for example, from 300 mesh to 400 mesh,such as from 325 mesh to 380 mesh and the mesh wire, which can bestainless steel, can have a diameter from about 0.3 mils to 1.5 mils,such as a diameter from 0.5 mils to 1.1 mils. Other screens and meshsizes can be used as appropriate for a particular metallization paste,process conditions, and desired feature sizes.

The deposited metallization paste in the form of electrical conductorssuch as grid lines can have, for example, a width from 0.5 mils to 4mils, and a height from 0.1 mils to 1.5 mils.

After being applied to a Si substrate, the screen-printed compositioncan be dried, for example, at a temperature within a range from 200° C.to 400° C. for from 10 seconds to 60 seconds, and then baked and firedat a temperature within a range from 300° C. to 500° C. For example peaktemperature can range from 400° C. to 500° C., and the time of exposureto heating can range from 30 seconds to 50 seconds, to providefront-side electrical conductors. Other temperatures, times, andtemperature profiles may be used. In general, it can be desirable thetemperature be less than 500° C.

Solar cell busbars having dimensions of 1.2 mm width and 16 μm heightcan exhibit and electrical resistivity of 1.9 μΩ-cm and can exhibit anadhesion strength of at least 2 N on a Si substrate, where theelectrical conductivity is determined according to line resistivityelectrical probe measurement and the adhesion strength is determinedaccording to a 180° solder tab pull test. For context, Ag thick-filmbusbars having a resistivity less than 2 μΩ-cm and an adhesion strengthgreater than 1.5 N are generally considered acceptable for use in thesolar cell industry.

Solar cell conductive electrodes prepared from metallic glass-coatedparticles provided by the present disclosure maintain acceptableconductivity and adhesion strength following exposure to acceleratedenvironmental test conditions including damp-heat testing andaccelerated thermal cycling, which are used to qualify solar cells for a25-year service life.

According to an aspect of the invention, a method of interconnecting asilicon substrate, comprises forming a positively charged oxide tunneljunction on a first region of a back surface of a silicon substrate;forming a negatively charged oxide tunnel junction on a second region ofa back surface of a silicon substrate; forming a positive electrode onthe positively charged oxide tunnel junction; and forming a negativeelectrode on the negatively charged oxide tunnel junction.

According to any of the preceding aspects, the method comprises, beforeforming the charged oxide tunnel junctions, forming a doped layer on afront surface of a silicon wafer, wherein the silicon wafer comprises asilicon substrate comprising a front surface and a back surface oppositeto the front surface; depositing an antireflection layer on the dopedlayer on the front surface; and planarizing the back surface.

According to any of the preceding aspects, forming the positiveelectrode and forming the negative electrode comprises screen printing ametallization paste on the positively charged oxide tunnel junction andon the negatively charged oxide tunnel junction.

According to any of the preceding aspects, the metallization pastecomprises: copper particles and metallic glass; copper particles andmetallic glass-coated copper particles; or metallic glass-coated copperparticles.

According to any of the preceding aspects, the metallization pastecomprises: silver particles and metallic glass; silver particles andmetallic glass-coated silver particles; or metallic glass-coated silverparticles.

According to any of the preceding aspects, the positively changed oxidetunnel junction and the negatively charged oxide tunnel junction form aninterdigitated pattern.

According to any of the preceding aspects, the positive electrode andthe negative electrode form an interdigitated pattern.

According to any of the preceding aspects, forming the positivelycharged oxide tunnel junctions comprises screen printing a positivelycharged dielectric paste.

According to any of the preceding aspects, the positively chargeddielectric paste comprises silicon oxide.

According to any of the preceding aspects, the cured positively chargedoxide tunnel junction is characterized by a thickness less than 50 Å.

According to any of the preceding aspects, forming the negativelycharged oxide tunnel junctions comprises screen printing a negativelycharged dielectric paste.

According to any of the preceding aspects, the negatively chargeddielectric paste comprises aluminum oxide.

According to any of the preceding aspects, the cured negatively chargedoxide tunnel junction is characterized by a thickness less than 50 Å.

According to any of the preceding aspects, forming the positivelycharged oxide tunnel junction and forming the negatively charged oxidetunnel junction comprises simultaneously forming the positively chargedoxide tunnel junction and forming the negatively charged oxide tunneljunction.

In an aspect according to the present invention, a solar cell comprisesinterconnects is fabricated using the method according to the presentinvention.

In an aspect of the present invention, a solar cell comprises a backsurface comprising a first region and a second region; apositively-charged silicon oxide layer overlying the first region; anegatively-charged aluminum oxide layer overlying the second region; apositive electrode grid overlying the positively-charged silicon oxidelayer; and a negative electrode grid overlying the negatively-chargedaluminum oxide layer.

According to any of the preceding aspects, the positively-chargedsilicon oxide layer and the negatively-charged aluminum oxide layer areinterdigitated; and wherein the positive electrode grid and the negativeelectrode grid are interdigitated.

According to any of the preceding aspects, each of thepositively-charged silicon oxide layer and the negatively-chargedaluminum oxide layer are characterized by a thickness from 1 Å to 45 Å.

According to any of the preceding aspects, each of the positiveelectrode grid and the negative electrode grid comprise copperparticles, copper alloy particles, silver particles, silver alloyparticles, or a combination of any of the foregoing. According to any ofthe preceding aspects, each of the positive electrode grid and thenegative electrode grid comprise amorphous metal glass.

It should be noted that there are alternative ways of implementing theembodiments disclosed herein. Accordingly, the present embodiments areto be considered as illustrative and not restrictive. Furthermore, theclaims are not to be limited to the details given herein, and areentitled their full scope and equivalents thereof.

What is claimed is:
 1. A method of forming a semiconductor deviceinterconnect, comprising: depositing a positively-charged oxide layer ona first region of a silicon substrate and forming a positively chargedoxide tunnel junction region on the first region of the siliconsubstrate; depositing a negatively-charged oxide layer on a secondregion of the semiconductor device and forming a negatively chargedoxide tunnel junction region on the second region the silicon substrate;depositing a metallization paste directly on the positively chargedoxide tunnel junction region; and depositing a metallization pastedirectly on the negatively charged oxide tunnel junction region, forminga positively charged oxide tunnel junction region and forming anegatively charged oxide tunnel junction region to form a positiveelectrode and a negative electrode; wherein the metallization pastecomprises: a metal, a metal alloy, or a combination thereof; and ametallic glass.
 2. The method of claim 1, wherein the positively chargedoxide layer comprises silicon oxide.
 3. The method of claim 1, whereinthe negatively charged oxide layer comprises aluminum oxide.
 4. Themethod of claim 1, wherein the metallic glass comprises ZrCuNiAl, CuZr,CuZrAl, CuZrAg, CuZrAlAg, CuZrAlAgNi, CuZrAlNi, ZrTiNbCuNiAl, MgZn,MgCu, MgNi, MgNiY, MgCa, AlNiYLa, AlMg, AlMgCa, TiZrCuNi, NiNbZrTiPt,AlNiLa, ZrCuTiNiBe, MgCuYAg, AlLiCu, AlYFe, AgMgCa, AgMgCaCu, or acombination of any of the foregoing.
 5. The method of claim 1, wherein,the metallic glass comprises an amorphous content from 50 wt % to 99.9wt %, wherein wt % is based on a total weight of the metallic glass. 6.The method of claim 1, wherein the metallic glass comprises acrystalline content from 1 wt % to 50 wt %, wherein wt % is based on atotal weight of the metallic glass.
 7. The method of claim 1, whereinthe metal or metal alloy comprises copper, zirconium, aluminum, silver,nickel, or a combination of any of the foregoing.
 8. The method of claim1, wherein the metal or metal alloy comprises silver, a silver alloy,copper, a copper alloy, or a combination of any of the foregoing.
 9. Themethod of claim 1, wherein depositing the positively charged oxide layerand depositing the negatively charged oxide layer independentlycomprises depositing a layer having a thickness from 1 Å to 45 Å. 10.The method of claim 1, wherein forming a positively charged oxide tunneljunction region and forming a negatively charged oxide tunnel junctionregion comprises exposing the deposited positively charged oxide layerand exposing the deposited negatively charged oxide layer to atemperature up to 450° C.
 11. The method of claim 1, wherein forming thepositive electrode comprises exposing the deposited a positive electrodemetallization paste overlying the positively charged oxide tunneljunction region to a temperature from 300° C. to 500° C.
 12. The methodof claim 1, wherein forming the negative electrode comprises exposingthe deposited negative electrode metallization paste overlying thenegatively charged oxide tunnel junction region to a temperature from300° C. to 500° C.
 13. The method of claim 1, wherein the metallic glasscomprises metallic glass particles, metallic glass-coated metalparticles, or a combination thereof.
 14. The method of claim 13, whereinthe metallic glass-coated particles comprise a metallic glass coatingsurrounding a core.
 15. The method of claim 14, wherein the corecomprises silver, a silver alloy, copper, or a copper alloy.
 16. Themethod of claim 15, wherein the core silver particles, copper particles,or a combination thereof.